In general, a transformer is a device that comprises a core (e.g., magnetic), a primary coil and one or more secondary coils. The primary coil receives electrical energy from a power source and couples the energy to the secondary coil(s) by virtue of changing magnetic field, wherein the energy appears as an electromagnetic field across the coil. If a load is connected to the secondary coil, the energy is transferred to the load. The output power of the transformer cannot exceed the input power to the transformer, so the output current is reduced in direct proportion to the gain in voltage (and vice versa).
RF (radio frequency) transformers are commonly used in electronic circuits for applications such as impedance matching (for maximum power transfer), AC voltage step-up/step-down, DC isolation between two circuits, common mode rejection, filters, etc. In addition, a transformer can be used to construct a BALUN having, e.g., a balanced input, where both input ports are isolated from ground to an unbalanced output where one output port is connected to ground.
For on-chip applications, transformers are typically constructed using coupled wires. A simple transformer structure comprises two wires with the same windings on each side, which is referred to as a 1:1 transformer or simply coupled-wires. By way of example, FIG. 1 is a perspective view of a semiconductor device having a conventional integrated transformer device. In FIG. 1, the semiconductor device (10) comprises a substrate (11) having integrated coplanar transformer (12) formed on a surface thereof. The transformer (12) comprises a first conductor (13) (primary) and a second conductor (14) (secondary) that are disposed parallel on the same layer. The conductors are patterned from a metal layer that is formed on the substrate surface, and then encapsulated in a dielectric or insulating layer.
Transformer devices such as depicted in FIG. 1 typically have poor electrical performance (e.g., low coupling, k=0.06) and exhibit high loss, especially when implemented for high frequency applications. Indeed, for lossy substrates such as silicon, the capacitive coupling between the metal lines (13), (14) and substrate (11) can result in increased power dissipation. If the metal lines are reduced in width to limit such capacitive coupling, the resistance of the metal line increases (e.g., via skin effect) resulting in increased power dissipation. Moreover, conventional transformer designs such as in FIG. 1 do not have well-defined return paths for closed environment EM conditions, which results in the electrical performance being more sensitive to surrounding metallic components. According, integrated circuit coplanar transformer devices such as depicted in FIG. 1 are typically used at lower frequencies where lower coupling factors, losses due to the skin effect, and inaccuracies caused by model to hardware discrepancies can be tolerated.